1. Field of the Invention
The present invention relates to a method of controlling an inkjet printhead containing a substantially closed duct in which ink is situated, said duct having an exit opening for the ejection of ink, wherein an actuation pulse is applied to an electromechanical transducer so that the pressure in the duct changes in such a manner than an ink drop is ejected from the exit opening. The present invention also relates to an inkjet printhead suitable for applying the present method and an inkjet printer containing such a printhead.
2. Related Art
A method of this kind is known from EP 0 790 126. The known method is used in a printhead for an inkjet printer, in which the printhead contains a duct plate in which a number of parallel grooves are formed in a longitudinal direction, each groove terminating in an exit opening or nozzle. The duct plate is covered by a flexible plate so that the grooves form substantially closed ink ducts. A number of electromechanical transducers are provided on the flexible plate at the ducts so that each duct is confronted by one or more of these transducers. The transducers, in this case, piezo-electric transducers, are provided with electrodes. When a voltage is applied in the form of an actuation pulse across the electrodes of the piezo-electric transducer of this kind, a sudden deformation of the transducer occurs in the direction of the associated duct, so that the pressure in that duct increases suddenly. As a result, a drop of ink is ejected from the nozzle.
On the side remote from the duct plate, the transducers are supported by a carrier member. The printhead is also provided with a number of connecting elements which connect the carrier member via the flexible plate to the duct plate. These connecting elements serve to increase the mechanical strength of the printhead so that an applied actuation pulse will also always result in the required pressure rise and thus the required drop ejection, i.e. a drop ejection in which the drop, for example, has a previously known size and/or previously known speed.
The use of the known method in known printheads therefore leads to a stable printing process.
The known method has a number of disadvantages however. Firstly, no matter how rugged the construction of a printhead, it will always age. Not only will material properties and particularly the expansion characteristic of the electromechanical transducer slowly change in the course of time, but the mechanical construction itself is also subject to change. Thus connections between the different constituent parts of the printhead, particularly glued connections, may acquire different mechanical properties or even bercome detached. All this has the result that a specific actuation pulse will in the course of time give a different drop ejection. In other words, the known method results in a decline in print characteristics.
Another disadvantage of the known method is that the maximum frequency at which drops can be ejected is limited. A subsequent drop cannot be ejected until the pressure change as a result of the previous drop has sufficiently decayed. Actuation of the transducer in fact usually results in a pressure change in the form of a damped sine wave. Only when the sine wave has been sufficiently damped will it not have an adverse effect on the next drop formation. This damping takes time and thus limits the maximum attainable drop frequency and thus restricts the maximum attainable print speed possible with the known method.
Another disadvantage of the known method is that cross-talk still occurs between the ducts. Although it is limited, particularly in applications where a very high quality is required, it is a significant disadvantage. Finally, it is a disadvantage that the known method requires the use of a printhead having little freedom with respect to design. The construction must satisfy strict mechanical requirements to provide a reliably stable drop formation. This makes it difficult and particularly expensive to use the known method.